1. Technical Field
The disclosed hybrid electric vehicle has an engine and a battery powered motor for delivering power to a multiple-ratio transmission for powering vehicle traction wheels.
2. Background Art
It is known design practice to use a combination of an internal combustion engine with an electric motor in a hybrid electric vehicle powertrain to provide the power needed at the vehicle traction wheels. Such vehicle powertrains will improve engine fuel economy compared to the fuel economy of a conventional vehicle with a powertrain that does not include an electric motor.
One example of such a hybrid electric vehicle powertrain is referred to as a modular hybrid transmission (MHT) that has a parallel hybrid architecture. The present invention may be used in a MHT, but it is not limited in use to a MHT. It may be applicable also to other known hybrid electric vehicle powertrains, such as a series hybrid powertrain or a power split hybrid powertrain. The latter powertrain establishes two power flow paths, one path delivering engine power and the other path delivering electric motor power to vehicle traction wheels.
The modular hybrid transmission presently described is a pre-transmission, parallel, hybrid powertrain with a clutch positioned between an electric motor and an engine. The clutch, which may be referred to as a disconnect clutch, may be fully integrated into a hydraulic control system for a multiple-ratio transmission. Further, it may be actuated by a linear solenoid, sometimes referred to as a variable force solenoid (VFS). The VFS is controlled by a geared transmission electronic control module (TCM).
In a non-hybrid conventional powertrain, there is a requirement known as top gear gradability. This requirement is the ability to maintain a given speed in top gear at 65 mph, for example, assuming that there are no accessory loads on the engine, such as an air conditioning compressor, and that there is no head wind. Typically, this requirement would be the ability to maintain a top gear on a 4.5% uphill grade. A low gradability requirement can improve highway fuel economy, whereas a high gradability requirement would improve drivability. A desirable gradability can reduce the amount of “shift busyness” whereby frequent upshifts and downshifts occur because of changes in road grade or because of small, transient changes in driver demand for torque at the wheels. An undesirable gradability may be apparent as a so-called “hunting” that occurs while traveling on a hilly terrain.
The transmission used in the modular hybrid transmission architecture herein described has multiple ratios, and the hydraulic transmission control system and the electronic transmission control system will achieve ratio changes between a high ratio and a so-called low ratio. Such changes will be referred to as upshifts or downshifts.
In certain hybrid electric vehicle powertrains, a so-called hydrokinetic torque converter is situated between the power output side of the engine and the power input side of the geared transmission. A torque converter includes an impeller driven by the engine and a turbine, which delivers power to a power input shaft for the transmission. A toroidal fluid flow circuit is established in the torque converter as engine power is delivered to the impeller and a toroidal fluid flow in the torque converter fluid circuit establishes a hydrodynamic torque. A stator or a reactor is disposed between the turbine fluid flow exit and the impeller fluid flow entrance. The stator changes the tangential direction of the toroidal fluid flow as the flow enters the impeller whereby a torque multiplication occurs due to a moment of momentum in the toroidal fluid flow path through the torque converter.
A torque converter lock-up clutch may be used to directly connect the impeller and the turbine for the torque converter after the torque converter achieves a speed ratio close to unity at a so-called clutch point. When the clutch point is achieved, a hydrodynamic fluid slip between the turbine and the impeller will occur. The lock-up clutch, as it is engaged, will reduce the slip to zero when torque multiplication at the converter is not required. This condition is known as a converter lock-up. For purposes of this disclosure, a converter lock-up may be considered to be a form of a transmission upshift.
A plot, which may be referred to as a downshift schedule, will illustrate a relationship between vehicle speed and a driver demand for torque, the latter being indicated by an accelerator pedal position. The plot for illustrating the shift point characteristics of a transmission upshift or a lock-up of the converter lock-up clutch will differ from the corresponding characteristics for a downshift or a converter unlock.
If the spacing requirements on the plot for an upshift and a downshift are too close, a so-called “hunting” may occur between the upshift and downshift states. This hunting can occur on a hilly and rolling terrain, for example. As the driver moves the accelerator pedal to demand additional output torque on an uphill slope, that typically would be followed by a back-off of the accelerator pedal if the driver wishes to maintain a constant speed on a downhill slope. As explained above, if the upshift and the downshift curves, or if the lock and unlock curves, are too close together, the changes in pedal position due to small transient changes in torque demand may cause the hunting condition to occur.